The Minister of Transport Phil Twyford has been fined $500 for using his cell phone on an aircraft after the doors had closed.

In May, Mr Twyford apologised for using his phone after the doors had closed on a flight. He offered his resignation, which wasn't accepted, and transferred responsibility for the Civil Aviation Authority to Associate Transport Minister Julie-Anne Genter.

Acting Prime Minister Winston Peters said Mr Twyford would be reinstated with responsibility for the CAA.

"I reiterate my unreserved apology for using my phone after the aircraft doors had shut in preparation for take-off," Phil Twyford said on Monday.

"This is inappropriate for anyone, but particularly inappropriate for me as Transport Minister.

"I accept the findings of the investigation that I breached Civil Aviation Authority rules and will pay the $500 infringement fine."

Director of Civil Aviation Graeme Harris says while Mr Twyford's actions did not pose a significant risk to the flight, he did breach the rule.

"It's imperative that passengers comply with Civil Aviation Rules and follow the instructions of cabin crew at all times," he says.

The law says portable electronic devices are not to be operated on board in breach of rules. The maximum fine for that is $2500.